In mobile phones, portable computers and other portable electronic products a lithium secondary battery is usually used as power supply to replace alkaline secondary batteries such as nickel-hydrogen and nickel-cadmium secondary batteries. When short-circuit occurs in the circuit outside the battery, a large short-circuit current will pass through the battery, which may cause the battery to become hot and scalding to the touch and even may cause an explosion. In order to avoid the above-mentioned dangers, the current should be cut off when short-circuit happens in batteries. The prior art discloses some methods for this purpose. For example, the Chinese Patent Application No. CN1197534A discloses a battery utilizing a positive temperature coefficient (PTC) element. The resistance of the PTC element is small during the normal charge. In case the battery is short-circuited, a large current will pass through the PTC element and the temperature of the PTC element will rise drastically in a short time, and its resistance will rise all of sudden when its temperature rises to a certain value and therefore the circuit is close to the status of being broken off. After the temperature of the PTC element drops, its resistance will goes back to its normal low level, and the circuit will become conductive once again. By adopting this kind of PTC element, the battery can be cut off promptly from the short-circuited loop. Besides, the PTC element is resistant to the impact of short-circuit current many times. The Chinese Patent Application No. CN1177217A discloses a battery, which has a protective circuit to prevent the damage caused by the external short-circuit. This protective circuit may detect the status of the battery. When the battery is cut off from the electronic device to which it is connected, the protective circuit will detect the breakaway of the external circuit and therefore will cut off the connection with the external power to avoid the short-circuit. By utilizing this kind of protective circuit, short-circuit of the battery caused by short-circuit of external powers can be fully avoided. The Chinese Patent No. ZL01235497X discloses an alkaline secondary battery having the function of short-circuit protection. In this kind of batteries, the conducting terminal of the electrode is made of nickel, tin, copper or lead. A default portion, such as a narrow neck, pore, mesh or the combination thereof is provided on the conducting terminal. The terminal will be burnt off within 0˜2 seconds when a short-circuit current larger than 30 A passes through it, thus the dangers, such as overheat, explosion and electrolyte leakage caused by a long-term short-circuit of batteries can be avoided.
The research of the inventors of the present invention has revealed that the above techniques disclosed in the above patent applications and patent are applied mainly to nickel-cadmium battery and nickel-hydrogen battery used in electric toys and tools, and they show the following disadvantages when they are applied to lithium secondary batteries: (1) the reliability of the safety protection will be decreased because the protection is realized by utilizing the physical properties of materials as used; (2) the protection device occupies a definite external space, and therefore the corresponding battery capacity will be reduced in relation to the battery with the same size.